The present invention relates to a new and improved construction of an apparatus for, and a method of, automatic gain/offset control of a video camera.
The invention thus generally relates to the fields of optics and electronics and, specifically, to the field of image processing techniques. In its more particular aspects the present invention specifically relates to a new and improved construction of an apparatus for, and a method of, automatic gain/offset control of a video camera for automatically optimizing the brightness and the contrast of a video image which is used for controlling the adjustment of a manipulator unit. In particular, the video camera is utilized for controlling the operation of an industrial robot. The video camera also may constitute a FLIR camera, i.e. a forward looking infrared camera and may be connected to an image control unit constituting a so-called video tracker in an automatic target tracking device. An infrared camera which is sensitive to heat radiation, is conventionally designated as a forward looking infrared camera.
Means for optimizing the brightness distribution in images are well-known. Thus, in modern photo cameras, the film exposure is optimized, for example, either by automatically adjusting the aperture at a predetermined exposure or exposure duration or by means of a programmable optically controlled exposure time-aperture combination. In such systems, the required brightness data are obtained either from the mean brightness value of the entire image, the brightness of a small pre-selected section or frame of the image by means of a so-called spot measurement, or from a comparison of a predetermined number of partial image sections or frames. This can be derived from the data sheets of respective photo cameras.
When using cameras employing electronic image recording such as, for example, video cameras, there can not only be optimized the brightness by means of the offset control but also the image contrast may be subjected to an optimizing operation. For this purpose there is varied, in addition to the offset, a second parameter, namely the gain which corresponds to a displacement of the image brightness distribution along the brightness axis. In such case, reference is made to an automatic gain/offset control.
Conventionally, the gain/offset control in video cameras is adjusted to provide an image which is as attractive as possible and as rich in information as possible for the human eye.
Hitherto and, for example, with respect to robotics, the gain/offset control was used in connection with video cameras for the recognition or detection of objects and for controlling related manipulator units. In such uses, the gain/offset control adjustment was optimized, if optimized at all, in accordance with the criterion of good visual recognizability. Thus the image was adapted in an optimum manner with respect to the eye of the machine operator but not with respect to the mode of operation of the control device connected with such video camera for controlling the manipulator unit.
In the same manner the gain/offset control adjustment was carried out in connection with video cameras utilized for target tracking, in accordance with optimum visual recognizability. This means that the video camera like, for example, the aforementioned forward looking infrared camera was controlled in a manner such that no saturation was observed merely within a predetermined image section or frame. Although thereby there was favorably insured an additional visual target monitoring process, however, the image did not constitute in any case an optimum image for automatically tracking the target by means of the target tracking platform.
It will be apparent that the human eye and an electronic image evaluating unit place different requirements on the image quality. The reason therefore is that the physiological process of vision by the human eye is proportional to the logarithm of the light intensity whereas, during electronic image recording, the recorded signal is linearly dependent upon the light intensity in a first approximation.
Furthermore, and by virtue of the integral image perception by the human eye, a structure or object is relatively readily distinguished or discriminated with respect to an associated background even at a poor signal-to-noise ratio or poor contrast Contrary thereto, automatically operating structure or object recognition means can extract the information related to the object or structure from a comparison of adjacent image brightness values only at great expenditure and on the basis of a fairly exceptional or extensively devised algorithm. Consequently, it is highly significant that the brightness of the object to be recognized compares as favorably as possible with the brightness of the background associated with the object to be recognized.
Regarding the automatic gain/offset control of a video camera, it should be noted that it is not intended to generate an image which is as favorable as possible for the human eye. Instead, the image should be optimized with respect to further electronic processing operations particularly in connection with the control of the performance of the manipulator unit. In order to obtain such optimization, it is thus required that the object to be monitored by the video camera can be distinguished or differentiated from the associated background. Such precondition can be satisfied at least for some periods of time by objects which move relative to the associated background.